Any object emits radiation, the spectral characteristics of which depend on the object's temperature. If the temperature of the object is different from that of ambient, radiative energy transfer occurs between the object and its surroundings. This energy transfer can be detected by an appropriate sensor. A warm, moving object (a person or animal) not only radiates electromagnetic energy, but its location with respect to the detector changes. In other words, the object can be differentiated from the surroundings by three factors: temperature, location and speed of moving. Therefore, detection must consist of the following steps: collection of radiation, conversion of the radiation into an electrical signal and signal processing.
Several types of infrared passive motion detectors are known in the prior art. Mortensen (U.S. Pat. No. 4,052,716) discloses a system with a plurality of thermistors which collect radiation by means of a parabolic focusing mirror. It uses capacitive coupling to distinguish between slow and fast moving objects. A similar detector, utilizing a thermopile sensor, is disclosed by Schwartz (U.S. Pat. Nos. 3,760,399 and Re. 29,082). In this invention, two fixed thresholds and an OR gate are used to detect positive and negative going signals from a preamplifier. The location and direction of the moving object in the prior art is determined by the above cited authors and by Keller (U.S. Pat. No. 4,052,616) and Schwartz (U.S. Pat. No. 3,958,118). All these detectors use either thermistors or thermocouples as discrete sensors.
The focusing systems in the prior art are made of lenses, parabolic mirrors and/or various reflectors to concentrate radiation on the individual sensors. Some detectors use multiple Fresnel lenses (facet) which focus radiation on a single pyroelectric or thermopile sensor (RCA Motion Switch C-23, Visonic Ltd. Motion Sensor SR-2000E; Kesser Electronics International, Inc. Infrared Sensor, Model 2006 and others presently commercially available). Instead of a small, solid-state sensor, a pyroelectric detector can be designed with a polymer film, such as polyvinylidene fluoride (PVDF). Cohen (U.S. Pat. No. 3,809,920) discloses a design which contains the polymer film with conductive electrodes on both surfaces. The heat flow from non-moving objects can be separated by a cancelling technique, as was disclosed in the U.S. Pat. No. 3,839,640 or by the use of a differential amplifier (Smith et al. U.S. Pat. No. 4,379,971). Smith's detector utilizes a polymer film with interdigitized (alternating) electrodes on one side and a parabolic mirror as a focusing system. The patent of Southgate (U.S. Pat. No. 3,842,276) discloses the use of an alternative electrode arrangement on the opposite surfaces of the film to produce ambient temperature compensation.